I just today stumbled upon the Code Metrics thing, after having used Visual Studio practically every day for... practically my whole life. I think it's been there since around VS2008, but I haven't cared to play with it.
Most of my projects in my solution have fairly high Maintainability Indices -- 85, 83, 86, and 76 and 59 on my "scrap" projects. I do remember putting very little effort into making those scrap projects presentable, so I agree with the index.
But I can't imagine it being a perfect measure. In what situations should I rightfully disagree with an index?
"A Practical Model for Measuring Maintainability" talks about the problems with the maintainability index. One of the main drawbacks is that it is a composite index of several other metrics, so it's hard to say why the score is bad.